Temperature Dependence of Competitive Reaction of Iodine Ions on H

Nov 11, 2008 - ... Toyonaka, Osaka 560-8531, Japan, Core Research for Evolutional Science and Technology (CREST), JST, Kawaguchi, Saitama 332-0012, ...
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J. Phys. Chem. C 2008, 112, 19005–19011

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Temperature Dependence of Competitive Reaction of Iodine Ions on H-Terminated Si(111) Surface in a Concentrated HI Solution Akihito Imanishi,*,†,‡ Takeshi Hayashi,† Kenta Amemiya,§,| Toshiaki Ohta,§ and Yoshihiro Nakato‡,⊥ DiVision of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan, Core Research for EVolutional Science and Technology (CREST), JST, Kawaguchi, Saitama 332-0012, Japan, Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, High Energy Accelerator Research Organization, Institute of Materials Structure Science, Tsukuba, Ibaraki 305-0801, Japan, and The Institute of Scientific and Industrial Research (ISIR), Osaka UniVersity, Ibaraki, Osaka, 567-0047, Japan ReceiVed: June 24, 2008; ReVised Manuscript ReceiVed: October 12, 2008

The surface reactions of iodine ions on nearly atomically flat, H-terminated Si(111) surfaces, when immersed in 7.6 M HI in a temperature range of -20 to 63 °C, were investigated by atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, and core-level shift spectra. It was revealed that the following two reactions simultaneously occurred, which were competitive with each other: (1) substitution reaction of Si-H bonds with Si-I bonds and (2) cluster formation by the condensation of SiHxI4-x or their oligomer species, which were generated by surface etching reaction. The ratio of those competitive reactions was strongly dependent on the temperature of the HI solution. At the high temperature, the former reaction was enhanced, whereas the latter reaction was suppressed. Inversely, at the low temperature, the latter reaction was relatively enhanced, resulting in that the ordered rodlike clusters were formed on the Si(111) surface. These results were explained by the fact that the former and latter reactions were mainly enhanced by the H2O and dissolved oxygen, respectively, and the concentration of dissolved oxygen was drastically decreased in the HI solution at high temperature, compared with that at low temperature. It was also revealed that the order and shape of the clusters formed by the condensation of SiHxI4-x or their oligomer species produced by etching reaction were strongly dependent on the temperature of the HI solution. Introduction The study of the semiconductor surface in aqueous solutions containing iodine ions is of much interest in view of the possibilities of high energy conversion efficiency of photoelectrochemical devices. The redox solution containing I- and (or) I2 has been used as the ideal electrolyte for the dye-sensitized solar cell1-8 and wet-type Si solar cell9-14 due to its appropriate equilibrium redox potential Ueq(I3-/I-) against the band energy of TiO2 or Si electrode. However, few studies about the surface properties of those semiconductor electrodes in I- solution are reported, though the chemical properties of the semiconductor surface may strongly affect the performance of the electrochemical cell. Especially, the performance of the Si electrode may be strongly affected by the temperature and the concentration of I- ions in the electrolytes, because of the some chemical species formed by the reaction between the Si surface and Iions in an aqueous solution. In our previous study, we found that the oriented nanorod clusters were formed on the H-terminated Si(111) surfaces (abbreviated as H-Si(111)) during immersion in 7.6 M HI at room temperature (∼25 °C).15,16 Interestingly, only round * To whom correspondence should be addressed. Fax: +81-6-6850-6237. E-mail: [email protected]. † Graduate School of Engineering Science, Osaka University. ‡ Core Research for Evolutional Science and Technology. § The University of Tokyo. | Institute of Materials Structure Science. ⊥ The Institute of Scientific and Industrial Research (ISIR), Osaka University.

nanodots were formed when the Si(111) was immersed in 7.6 M HI containing 0.05 M I2, contrary to the case of immersion in 7.6 M HI with no I2. X-ray photoelectron spectroscopy (XPS) analysis suggested that those clusters were composed of silicon iodides (such as SiHxI4-x or their oligomer), produced most probably by Si etching with HI.16 In addition, atomic force microscopy (AFM) inspection revealed that the immersion at a low temperature below about 30 °C led to the formation of long rod-shaped clusters, oriented in the 〈1j1j2〉 direction or equivalent, whereas the immersion at a high temperature above 30 °C led to the formation of circular dot clusters, with their size and shape changing abruptly at about 70 °C.16 It is shown experimentally that the formation of dot clusters at a high immersion temperature is explained on the basis of thermodynamics, whereas that of oriented rod clusters at a low temperature is explained by a kinetics-controlled mechanism. On the other hand, we investigated the chemical reactivity of Si-H (and SiH2) bonds at H- Si(111) surfaces immersed in hydrogen halide (HX; X ) Cl, Br, and I) solutions by using a Fourier transform infrared (FTIR) spectrophotometer, XPS spectra, and flat-band potentials.16-18 The decreases in the intensity of FTIR bands, together with the increases in the surface atomic (X/Si) ratios obtained from XPS spectra, clearly showed that Si-H bonds in the HX solutions changed to Si-X. The conclusion was supported by large positive shifts in the flat-band potential of Si(111). It was discussed that the formation of Si-X bonds occurs by nucleophilic attack of halide ions. Those results indicated that the following two kinds of reactions occurred simultaneously at the Si surface in HI aqueous

10.1021/jp805586y CCC: $40.75  2008 American Chemical Society Published on Web 11/11/2008

19006 J. Phys. Chem. C, Vol. 112, No. 48, 2008 solution: (1) the formation of clusters by the condensation of SiHxI4-x or their oligomer species produced by Si etching with HI and (2) the nucleophillic attack reaction in which Si-H bonds are substituted with Si-I bonds. Those reactions may be competitive with each other. However, the relationships in the mechanisms between those two kinds of reactions have not been clarified. In the present work, we investigated the simultaneous formation of clusters and substitution of Si-H bonds with Si-I bonds on the H-Si(111) surfaces when they were immersed in 7.6 M HI using high resolution core-level shift spectroscopy, FTIR, and XPS, with an emphasis placed on the elucidation of the temperature dependence of those two reactions. Especially, core-level shift spectroscopy using low energy and high intensity radiation emitted from synchrotron is a powerful method to investigate the oxidation state of the Si surface or adsorbed species in detail because this method has the advantage of high resolution and high surface sensitivity. The investigation of the effect of the temperature of HI solutions has revealed that the balance of those competitive reactions was drastically changed with the immersion temperature. Experimental Section Single crystal n-type Si(111) wafers having a resistivity of 10∼15 Ω · cm and a vicinal surface tilting in the 〈1j1j2〉 direction at an angle of 0.36 ( 0.1° were obtained from Shin-Etsu Handoutai Co. Ltd., Japan. Nearly atomically flat and Hterminated Si(111) surfaces with a step and terrace structure were obtained by the conventional RCA cleaning method,19 followed by etching with 5% HF for 5 min and with 40% NH4F for 15 min.20-27 The Si surfaces thus treated were then immersed in 7.6 M HI aqueous solution for certain periods of time in a range from 0 to 24 h. The Si immersion was carried out in the dark under a nitrogen atmosphere in order to prevent the oxidation of the Si surface by either photogenerated holes or oxidants in solution such as dissolved oxygen and I2 (or I3-). A double-walled glass vessel was used for the Si immersion, and the temperature of 7.6 M HI, put in the inner part of the vessel, was kept constant by circulating temperature-regulated water through the outside opening of the vessel. Chemical analysis of the Si(111) surface and adsorbed species was carried out with the Si-2p core level photoelectron spectra measurements; those were carried out at the soft X-ray doublecrystal monochromator station BL-7A of the Photon Factory (ring energy of 2.5 GeV and ring current of 350-250 mA) in the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK-PF). The absolute photon energy was calibrated with a sharp Au 4f-7/2 resonance at 84.0 eV.28 Si-2p core level spectra were obtained using a photon energy of 130 eV. The analysis chamber used for this measurement consisted of a load-lock sample transfer system and hemispherical analyzer (Scienta-SES200). The total energy resolution (monochromator and analyzer) was determined to be 100 meV at a photon energy of 130 eV. The polar angle θ of X-ray incidence was chosen as 90° (normal incidence). The X-ray beam and the hemispherical analyzer were fixed at an angle of 75°. FTIR spectra were obtained with a Bio-Rad FTS575C spectrometer, with a spectral resolution of 1 cm-1. The multiple internal reflection (MIR) method was adopted to get a high sensitivity. The infrared beam was focused at normal incidence on an edge (45-bevel) of the Si prism so that it was expected by calculation to be reflected internally about 100 times in the Si prism. Chemically oxidized Si wafers, used as the spectral

Imanishi et al.

Figure 1. Surface (I/Si) atomic ratio, as a function of the immersion time, for H-terminated Si(111) surfaces immersed in 7.6 M HI at -20, 5, 30, and 63 °C.

reference, were prepared by immersing in a boiling mixture of 30% H2O2 and 98% H2SO4 (1:1 in volume) for 5 min. Both of the compartments were purged with nitrogen gas or dry air. The amount of the adsorbed iodine species was obtained using an X-ray photoelectron spectrometer (Shimadzu ESCA-1000) having an Al KR line as the X-ray source. The surface atomic (I/Si) ratio was calculated from the integrated intensities of XPS I-3d and Si-2p peaks, with the contribution from the inside Si atoms of the Si crystal being corrected by a method of Himpsel et al.29 Surface morphology was inspected with an atomic force microscope (Digital Instruments NanoScope IIIa). The AFM images were obtained by tapping mode under an ex situ condition with the Si samples placed in air. The radius of an AFM tip was about 10 nm according to a catalogue of a production company (Nanosensors). Results Figure 1 plots the atomic (I/Si) ratio for the Si surface after the HI immersion at -20, 5, 30, and 63 °C as a function of the immersion time. The atomic (I/Si) ratio, obtained from XPS spectra, increased with the immersion time but showed a tendency of saturation in a range of the long immersion time. The results clearly indicate that the rate of Si etching with HI (and thus the rate of formation of the clusters) or the substitution reaction of Si-H with Si-I decreased with the immersion time. We can also see that the amount of the adsorbed iodine increased with increasing temperature of the HI solution, which indicates that the iodization reaction mentioned above is strongly enhanced in the solution at the high temperature. We checked whether the surface back-bond oxidation occurred or not at each point, by using the SiO2 peak (i.e., peak of Si4+) at 103 eV in the Si-2p XPS spectra. Although the spectra of the samples prepared at -20, 5, and 30 °C showed no oxidation even for 24 h immersion time, the sample prepared at 63 °C was slightly oxidized at immersion times over 12 h. Figure 2 shows the Si-2p core-level shift spectra of the H-Si(111) surface immersed in HI solutions at 5 °C for 0-24 h. A curve-fitting analysis of these spectra was performed, assuming the superposition of several resonances which were described by Gaussian peaks. The spectra were decomposed into four kinds of resonances. The most prominent ones, components A and B (0.18 eV shift with respect to peak A), are attributed to Si-2p (composed of Si2p-3/2 and Si2p-1/2) of bulk silicon and H-terminated silicon, respectively.30-36 The

Reaction of Iodine Ions on H-Si(111) Surfaces

Figure 2. Si-2p core-level shift spectra of the H-Si(111) surfaces immersed in 7.6 M HI aqueous solution at 5 °C for 0, 2, 12, and 24 h (black lines). The deconvoluted components of bulk Si, H-terminated Si, SiHxI4-x (or their oligomer) clusters, and iodine-terminated Si are shown by blue, green, pink, and orange lines, respectively, and the sum of them is shown by the red line.

component D (0.74 eV shift with respect to peak A) is assigned to monoiodine-terminated Si.37 The remaining component, peak C (0.36 eV shift with respect to peak A), may be assigned to the clusters consisting of iodine and silicon, and those are generated on the Si surface simultaneously with the formation of Si-I bonds.16 We can see that the intensity of peaks C and D increases as the immersion time increases. This is consistent with our previous report, in which the generation of Si-I bonds (i.e., substitution of Si-H with Si-I bonds) and rodlike clusters proceeds simultaneously with increasing immersion time in HI solution at 5 °C.16 No other peak was observed in the region from 100 to 103 eV, indicating that oxide (Si4+) or suboxide (Si1+∼Si3+) layers (appeared at 1.05∼3.58 eV shift with respect to bulk Si (peak A)) were not formed.30-36 Figure 3 shows the Si-2p core-level shift spectra of the H-Si(111) surface immersed in HI solutions at 5 °C for 2 h (a) and at -20 °C for 24 h (b) together with the spectrum of the H-Si(111) surface before HI immersion. Note that the amount of adsorbed iodine for both samples is almost the same (∼5%; derived from Si-2p and I-4d XPS). The deconvolution analysis revealed that, in the case of -20 °C, the component C was enhanced compared with that in the case of 5 °C. On the other hand, we observed the small peak of the component D for the spectrum of -20 °C, whereas that peak was enhanced in the case of 5 °C. This indicates that the formation of clusters was enhanced at -20 °C whereas the formation of Si-I bonds were enhanced at 5 °C. Figure 4 shows the FTIR spectra in a region of Si-H vibrations for the H-Si(111) surface immersed in HI solution at -20 °C for 24 h (b), at 30 °C for 1 h (c), and at 63 °C for 25 min (d). The spectrum of the H-Si(111) surface before HI immersion is also shown in (a) as a reference. Similarly to the case of Figure 3, the amount of adsorbed iodine is about 5%

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Figure 3. Si-2p core-level shift spectra of the H-Si(111) surfaces immersed in 7.6 M HI aqueous solution at 5 °C for 2 h (a) and -20 °C for 24 h (b) (black lines). The H-Si(111) surface before HI immersion is also shown. The deconvoluted components of bulk Si, H-terminated Si, SiHxI4-x (or their oligomer) clusters, and iodineterminated Si are shown by blue, green, pink, and orange lines, respectively, and the sum of them is shown by the red line. The amount of adsorbed iodine for both samples is almost the same (∼5%; derived from Si-2p and I-4d XPS).

for all the samples (b-d). Peak A appearing at 2083.6 cm-1 is attributed to the Si-H vibration at the terrace sites. A newly appearing peak at about 2087.0 cm-1 (peak B) is attributed to Si-H bonds (terrace) shifted by an interaction with Si-I bonds (terrace) produced at adjacent positions.16-18 (Note that the small peak originally appearing at around 2087 cm-1 in the spectrum (a) is attributed to Si-H bonds at step sites, which is different from newly the generated peak mentioned above.22,38,39) There is a possibility that the similarly shifted absorption peaks, assignable to Si-H vibrations of etching products, SiHxI4-x or formed clusters, were hidden or included in peak B, because the local structure around the Si-H bonds may be similar to that on the terrace sites. We can see that peak A is decreased with increasing temperature, whereas peak B is increased with increasing temperature, indicating that substitution reactions of Si-H with Si-I bonds were enhanced with increasing temperature. Peak B is blue-shifted more and more with the temperature increases. This may because the amount of the Si-I bonds (terrace) produced at adjacent positions of Si-H bonds increased with increasing temperature. Of course, we cannot exclude the possibility that the shift can be attributed to the fact that peak B also included the peak assigned to the shifted Si-H vibration in the clusters, which may be decreased with increasing temperature. Figure 5 shows the AFM images of H-Si(111) surfaces immersed in HI solution at -20 °C for 24 h (a), at 30 °C for 1 h (b), and at 63 °C for 25 min (c); those correspond to the spectra in Figure 4b, c, and d, respectively. In part (a), we can see that a large amount of rodlike clusters are formed on the surface, and those are aligned along the 〈1j1j2〉 and equivalent directions.15,16 In part (b), the amount of the clusters is drastically

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Figure 4. FTIR spectra for H-terminated Si(111) surfaces immersed in 7.6 M HI aqueous solution at -20 °C for 24 h (b), at 30 °C for 1 h (c), and at 63 °C for 25 min (d). The spectrum of the H-Si(111) surface before immersion is shown as a reference (a). The amount of adsorbed iodine for the all spectra is almost same with each other (∼5%; derived from Si-2p and I-4d XPS).

decreased. In addition, the order of the arrangement of the clusters decreased compared with those on the surface shown in part (a). In part (c), we can hardly see the clusters on the surface. Only the chevron-shaped step lines are seen on the surface, which is characteristic for the vicinal (111) surface tilting toward the 〈1j1j2〉 direction.16,40 Discussion The results of the core-level shift spectra (see Figure 3) indicate that the formation of Si-I bonds was enhanced at the high temperature. On the other hand, the results of FTIR and AFM measurements (Figures 4 and 5) indicate that the amount of remaining Si-H bonds at high temperature was smaller than that at low temperature, whereas the large amount of the clusters was formed at low temperature. These results indicate that the proportion of the reaction of the Si-I formation to cluster formation was drastically changed by the temperature of the HI solution. In other words, the former reaction was enhanced at high temperature and the latter one was enhanced at low temperature. Why was the proportion of the two competitive reactions changed by the temperature? We should consider the reaction model in HI aqueous solution to answer this question. The mechanisms of the etching reaction of a Si(111) surface in HF or NH4F aqueous solution are widely studied by many researchers.41-44 It is reasonable that we construct the reaction model for the HI aqueous solution on the basis of an established model for HF or NH4F aqueous solution. It is known that, in the case of etching reaction by HF or NH4F aqueous solution, the H2O molecules, dissolved O2 and F- ions play an important role in the reaction mechanism. In other words, the etching reaction proceeds with the surface oxidation by H2O and O2,

Imanishi et al. followed by the nucleophilic attack to positively charged Si by F- ions or water molecules. Thus, formed Si-F bonds on the surface are immediately changed to Si-H bonds by nucleophilic attack by water molecules. The successive occurrence of these reactions leads to the production of an atomically flattened and hydrogen-terminated Si(111) surface. In the case of HI solution, H2O and dissolved O2 may also play a same role as that in HF (or NH4F) solutions in the same manner. Thus, the key point to construct the mechanism for HI solution is the difference in the properties between F- and I- ions. On the other hand, the prominent difference in the experimental results between HF (or NH4F) and HI solutions is that (i) the clusters that are composed with etched species are formed on the surface in HI, whereas almost no clusters are observed on the Si surface in HF (or NH4F) solution and (ii) part of the Si surface was terminated with I in HI, whereas most surfaces are terminated with hydrogen in HF (or NH4F). These differences may come from the difference in the properties between F- and I- ions. Although those ions belong to the same group (halogen group), we should note that there is a difference in the electronegativity between them (Pauling electronegativities of Si, F, and I are 1.90, 3.98, and 2.66, respectively). This induces the difference in the polarization when those ions formed Si-X (X ) F or I) bonds, resulting in the suppression of the nucleophilic attack to the less polarized Si-I bonds, compared with Si-F bonds. First, as mentioned above, we assumed that the reactions in HI aqueous solution were initiated by the oxidation of the Si surface with water molecules and dissolved oxygen similarly to the case of HF (or NH4F) aqueous solutions. Figure 6 shows the plausible model for the reactions initiated by H2O (a) and dissolved O2 (b) at the Si surface in HI aqueous solution. (Note that this model is an example of a typical reaction. In an actual reaction, there are some other alternative reactions or mixed ones at each step.) The water molecules initially attack the Si-H species at the step sites,41 resulting in the formation of Si-OH, which is easily substituted with Si-I bonds by the successive nucleophilic attack of I ions. In the case of the H-terminated Si surface in HF aqueous solution, it is well-known that thus formed Si-F was easily changed to Si-H by the successive nucleophilic attack of H2O molecules due to Si-F polarization.42-44 However, in the case of the Si surface in HI solution, such a reaction may be suppressed because of the relatively less polarized Si-I bonds compared with that of Si-F bonds. Thus, the Si-I formed at the terrace sites may remain without further reaction (see Figure 6a). In other words, this difference in the Si-X (X ) F, I) polarization induced the difference (i) between HF and HI (i.e., Si-I bonds are formed only in HI solution) mentioned above. On the other hand, we can assume that the Si-I formed at step sites may be attacked by H2O molecules to form the Si-H(step) (i.e., step etching reaction proceeds), because the step sites are relatively unstable and more reactive compared with the terrace sites.41 This indicates that only the step sites are selectively etched under the condition in which only H2O and HI molecules are reactant. We should note that, in this model, the Si-I(ads) are formed only at the terrace sites that are newly generated by the successive etching of step sites (i.e., the Si-I were formed at the area where the top Si layer was removed by etching reaction). This is consistent with the fact that we could not observe diiodine but monoiodine-terminated Si from the core-level shift spectra. If the step sites were also terminated with iodine, diiodine-terminated Si (∼1.4 eV shift with respect to peak A)37 might also be observed, because there are large amount of dihydride sites on the step edge of the vicinal

Reaction of Iodine Ions on H-Si(111) Surfaces

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Figure 5. AFM images for H-terminated Si(111) surfaces after immersion in 7.6 M HI aqueous solution at -20 °C for 24 h (a), at 30 °C for 1 h (b), and at 63 °C for 25 min (c).

Figure 6. Plausible model for the reactions by H2O (a) and dissolved oxygen (b) at the Si surface in HI aqueous solution. Note that the two kinds of oxygen insertion reactions at the terrace and step sites shown in part (b) may occur simultaneously.

H-Si(111) surface tilting toward the 〈1j1j2〉 direction, whereas the terrace sites are monohydride sites.16 In addition, this is also supported by our previous results.16 We reported that FTIR and XPS spectra indicated that only the Si-H(ads) at the terrace sites were substituted with Si-I without decreasing the Si-Hx at the step sites. Furthermore, AFM inspection showed that the surface etching occurred selectively at step sites without changing the morphology of the terrace.16 Considering the fact that the late limiting step of the etching reaction is the oxidation of H2O,41 the above model is consistent with our previous results. On the other hand, it is well-known that the O atoms of dissolved oxygen are inserted into Si-Si back-bonds without removing Si-H termination.41 This induces the successive nucleophilic attack of I- ions due to its polarization of Si-O bonds. In addition, the insertion of O atoms caused the large distortion at the Si surface, resulting in the more drastic etching of the Si surface accompanied with the generation of the SiHxI4-x or their oligomer species. Thus, formed species will be condensed during the surface diffusion, which leads to the formation of clusters. Although similar species (SiHxF4-x) may be formed in HF (or NH4F) solutions, it is known that the SiHxF4-x (it is easily changed to SiHxOH4-x by successive attack of H2O molecules) is soluble in an aqueous solution. This caused the difference (ii) (i.e., the clusters were observed only in HI solution) mentioned above. In addition, we should note that the attack of the dissolved oxygen is less anisotropic than that of the H2O molecule,41 resulting in that not only step sites but also terrace sites may be etched by the insertion of O atoms and successive attack of

HI molecules. In an actual reaction, the above two competitive reactions simultaneously occur at the Si surface in HI aqueous solution. On the other hand, the amount of dissolved oxygen is drastically changed by the temperature of the aqueous solution. Takahagi et al. reported that the amount of dissolved oxygen in aqueous NH4F solution was reduced to 30% by increasing the temperature from 20 to 80 °C.45 Thus, in the case of the high temperature, the reaction of the dissolved oxygen (i.e., surface etching) was suppressed, whereas it was enhanced in the solution at the low temperature. This induced the large difference in the morphology of the Si surface formed by NH4F etching. The Si etched in NH4F solution at high temperature gave the smooth and flat surface, whereas the surface etched at low temperature is roughened and had a large amount of pits. Such a difference was also explained by the difference in the etching mechanism between H2O and dissolved oxygen.41 In the present study, the concentrations of dissolved oxygen of HI solution at -20 and 63 °C were 2.3 and 0.42 mg/L, respectively. Such a large difference in the concentration may also induce the difference in the chemical reaction; that is, the H2O reaction shown in Figure 6a is major at high temperature, whereas the O2 reaction shown in Figure 6b is enhanced at low temperature. This resulted in that the I- ions were consumed by the formation reaction of Si-I(ads) at high temperature, whereas they were consumed by the formation of iodine clusters at low temperature. We should note that the etched fragments SiHxI4-x (or their oligomer) should be also generated not only by the reaction of dissolved oxygen but also by the reaction of H2O, because H2O

19010 J. Phys. Chem. C, Vol. 112, No. 48, 2008 also etches the step sites on the surface. This indicates that the clusters may also be formed in the solution at high temperature. However, at least from AFM images shown in Figure 5, we observed no clusters on the Si surface at high temperature. On the other hand, we previously reported that the iodine clusters were formed on the Si surface in the HI aqueous solution even at 63 °C, if it was immersed in HI solution for a longer time (at least 60-240 min), though the formed clusters were dotlike in shape and the density of them was quite low,16 in contrast to the rodlike clusters observed at low temperature. Thus, these results indicated that, although the SiHxI4-x species may be also generated even at high temperature, the amount of them is quite smaller than that at low temperature. This is attributed to the fact that the etching by H2O molecules is more isotropic than that by dissolved oxygen, which leads to the site selective and moderate etching.41 In fact, we observed a small amount of condensed dotlike clusters on some parts of the Si surface for the immersion time of 25 min (not shown). The rod clusters formed at -20 °C are aligned in particular directions, that is, in the 〈1j1j2〉 direction and equivalents. This fact also strongly suggests that the formation of the rod clusters is controlled by kinetics. The formation of oriented needles (or their aggregates) is well-known for diffusion-limited aggregation (DLA).44,45 A typical example is crystals of snow. It is thus reasonable to assume that the formation of the oriented rod clusters at -20 °C is caused by a similar diffusion-controlled mechanism. The rodlike clusters formed at -20 °C were ordered fairly well. On the other hand, though a smaller amount of clusters also formed at 30 °C, it was obvious that the order and shape of them is worse and more symmetric (i.e., dotlike shape) compared with those at -20 °C. Although the order of those clusters formed at 30 °C became slightly better after longer immersion time by condensing each small cluster,16 the formed clusters at 240 min immersion time were also disordered compared with those at -20 °C. These results strongly suggest that the formation of clusters at high immersion temperature is mainly controlled by thermodynamics, whereas that of oriented rod clusters at a low temperature is explained by a kineticscontrolled mechanism. One possible explanation is like that; in the case of -20 °C, the formed SiHxI4-x fragments might diffuse to the specific direction which was determined by a surface atomic structure or a distribution of surface potential, resulting in the ordered rodlike clusters which reflect the structural symmetry of the Si(111) surface. However, a characteristic structure controlling the diffusion direction (e.g., surface troughs running along the specific direction) was not clearly found along the 〈1j1j2〉 and equivalent directions on the Si(111) surface, though a detailed analysis such as computer simulation is needed to clarify the diffusion manner. Thus, even in the case of low temperature, we cannot completely exclude the possibility that the thermodynamic factor (e.g., anisotropic strain at the interface between clusters and Si substrate) partly contributes to the formation manner of clusters. On the other hand, in the case of 30 °C, the thermodynamic factor such as the surface energy became prominent, resulting in that the more disordered and symmetrical shaped clusters were formed on the surface. Anyway, the details of the mechanism for the asymmetrically ordered rodlike clusters are under investigation. What is the detailed chemical component of the clusters? From the core-level shift spectra, we can see that the chemical shift of the clusters is smaller than that of monoiodine-terminated Si at terrace sites. This result contradicts the fact that the clusters

Imanishi et al. are formed with SiHxI4-x or their oligomer fragments. However, if we assumed that (1) the iodine included in the clusters is quite small (i.e., most of Si in the clusters are monoiodineterminated Si) and (2) the bonds in the clusters have large strain (Some researchers reported that negative shift for the Si component is induced by the changes of Si-O bond angles and bond lengths.48-50 It is quite possible that a similar phenomenon is observed in the case of Si-I bonds. In addition, it is wellknown that the clusters of Si compounds such as polysilane have large internal strain.), we can explain the present result. However, we need further investigation to prove this hypothesis and to elucidate the detailed chemical components and structures. Finally, it is interesting to consider what will occur on the Si(100) surface, which is widely used in the semiconductor industry in similar experimental conditions. It is expected that the fundamental two kinds of reactions (termination with iodine and surface etching) will also occur on the (100) surface, because those reactions themselves occur irrespective of surface structure. However, we should note that the temperature dependence is strongly dependent on the difference in the etching manner between H2O and dissolved oxygen. On the other hand, in the case of the Si(100) surface, it is well-known that an atomically flat surface is difficult obtain by etching with NH4F aqueous solution, probably because the anisotropy etching reaction by water molecules does not proceed efficiently. The lack of anisotropy in the etching manner of water molecules leads to the small difference in the etching manner between H2O and dissolved oxygen. Thus, we suppose that the distinct temperature dependence will not be observed on the H-Si(100) surface. Conclusion The surface reactions of iodine ions on nearly atomically flat, H-terminated Si(111) surfaces, when immersed in 7.6 M HI in a temperature range of -20 to 63 °C, was investigated by AFM, FTIR, and Si-2p core-level shift spectra. It was revealed that the following two reactions simultaneously occurred, which were competitive with each other: (1) substitution reaction of Si-H bonds with Si-I bonds and (2) cluster formation by the condensation of SiHxI4-x or their oligomer species, which were generated by surface etching reaction. Interestingly, the ratio of those competitive reactions was strongly dependent on the temperature of the HI solution. At the high temperature, the former reaction was enhanced, whereas the latter reaction was suppressed. Inversely, at the low temperature, the latter reaction was relatively enhanced, resulting in that ordered rodlike clusters were formed on Si(111) surface. Those results were explained by the difference in the concentration of dissolved oxygen between the high and low temperature HI solutions. It was revealed that the order and shape of the clusters formed by the condensation of the SiHxI4-x or their oligomer species produced by etching reaction were strongly dependent on the temperature of the HI solution. The arrangement and shape of them became more ordered and more asymmetric (i.e., rodlike shape) with decreasing temperature. This may be due to the fact that the arrangement of those clusters was controlled not by thermodynamics but by kinetics at low temperature. Acknowledgment. The present work has been performed under the approval of the Photon Factory Program Advisory Committee (PF PAC No. 2002G266). References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737.

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